Distribution and Ecology of Mesozooplankton (Northern and Central Adriatic Sea)
189
Conclusions
Our analysis of the space-time distribution of
zooplankton in the north and central Adriatic
neritic system revealed two sub-systems in the
late spring-summer period.. DefIned as coastal
and offshore zones, they exhibit different physico-chemical and biological features. The two
zones are separated by a frontal system, caused
by thermohaline stratifIcation of the water masses, with a pycnocline at about 10-15 m depth.
The zooplankton community of the neritic
system shows a space-time. variability in relation
to the different reproductive strategies and ecological niches of the species. From late spring to
summer, the zooplankton community of the
coastal zone exhibits high abundance and low
species diversity, as a result of the dominance of
a few species. The north-central Adriatic Sea is
characterized by a high inter-annual variability
of the zooplankton community (Benovic et al.
1984; Fonda Umani et al. 1992, 1994; Fonda
Umani 1996). The coastal zooplankton is characterized by the ecological association of opportunistic species of cladocerans (P. avirostris) and
copepods (A. clausi and P. parvus). Their reproductive rate increases contemporaneously with
the higher primary production in the springsummer period (Fonda Umani et al. 1992;
Zoppini et al. 1995; Scotto di Carlo et al. 1985);
thus they come to dominate the structure of the
coastal community.
The offshore zone is different from the
coastal one, both in the lesser abundance of zooplankton and the more equal distribution of
species composition. This is caused by the
reduced abundance of P. avirostris, A. clausi and
P. parvus and by the increasing numbers of C.
helgolandicus, C. vanus, O. plumifera and C. jobei.
The latter species are widespread mainly in
waters with smaller oscillations of the physicochemical parameters.
During late spring-summer, the copepod
population structure changes signifIcantly in
relation to seasonal changes in the dominance
ratio among A. clausi, P. parvus and T. stylifera.
Their distribution patterns show a certain degree
of temporal segregation of abundance peaks,
which would reduce the negative aspects of interspecifIc competition. In summer, the spatial distribution of P. elongatus and T. longicornis, typical euryoedous species of estuarine environments and well adapted to higher temperature
and lower salinity (Hure et al.1980), assume particular importance. Their wide distribution in
the neritic system depends on both the interaction of several biological and hydrological factors
and their vertical migration pattern. In this season, the abundance of P. elongatus and T. longicornis is higher and their distribution range
extends from coastal to offshore zones and from
north to south (Hure et al. 1980). Their marked
spreading into the offshore waters is caused by
the very strong outflow of the Po River in this
period (Marchetti et al. 1985). In the neritic system, the change from coastal hypertrophic surface waters to offshore oligo-mesotrophic ones
could cause the vertical migration to the bottom
of the above-mentioned species, either in the
same water layer with high fluorescence values or
in the NAdDW current which flows southward.
These observations on the spatial distribution pattern of P. elongatus and T. longicornis
suggest some considerations on the circulation
of water masses of the western Adriatic neritic
system. These species could be considered as
hydrological indicators both of the coastal waters
flowing south along the western coastal zone
(Hure et al.1980) and of the NAdDW, which originates in the area between the Po delta and Istria
and flows deeply from north to south in the offshore zone of the neritic system (Russo et a1. in
press).
During winter, the zooplankton community
has a dearly different pattern of species composition than in summer. The low abundance and
biomass of zooplankton are mainly due to the
lack of Penilia avirostris, Acartia cla us i,
Paracalanus parvus and Temora stylifera.
Copepods are the dominant zooplankton group,
even though they strongly decrease in number.
Moreover, there is an increased abundance of
pelagic species. which have a wide vertical distribution and are typical of sub-surface waters of
the southern Adriatic Sea. Their spreading
toward north-central areas is mainly related to
the seasonal ontogenetic migration to the surface
(Hure et a1. 1980). In this period, lower sunlight
and temperature values, together with greater
water turbulence, determine a strong reduction
of the primary production. The low primary production resources (Fonda Umani et al. 1992) are
used by a high number of species, so that the
copepod population structure is characterized
by a more equal distribution among the species
of the neritic system. No species becomes so
189
Conclusions
Our analysis of the space-time distribution of
zooplankton in the north and central Adriatic
neritic system revealed two sub-systems in the
late spring-summer period.. DefIned as coastal
and offshore zones, they exhibit different physico-chemical and biological features. The two
zones are separated by a frontal system, caused
by thermohaline stratifIcation of the water masses, with a pycnocline at about 10-15 m depth.
The zooplankton community of the neritic
system shows a space-time. variability in relation
to the different reproductive strategies and ecological niches of the species. From late spring to
summer, the zooplankton community of the
coastal zone exhibits high abundance and low
species diversity, as a result of the dominance of
a few species. The north-central Adriatic Sea is
characterized by a high inter-annual variability
of the zooplankton community (Benovic et al.
1984; Fonda Umani et al. 1992, 1994; Fonda
Umani 1996). The coastal zooplankton is characterized by the ecological association of opportunistic species of cladocerans (P. avirostris) and
copepods (A. clausi and P. parvus). Their reproductive rate increases contemporaneously with
the higher primary production in the springsummer period (Fonda Umani et al. 1992;
Zoppini et al. 1995; Scotto di Carlo et al. 1985);
thus they come to dominate the structure of the
coastal community.
The offshore zone is different from the
coastal one, both in the lesser abundance of zooplankton and the more equal distribution of
species composition. This is caused by the
reduced abundance of P. avirostris, A. clausi and
P. parvus and by the increasing numbers of C.
helgolandicus, C. vanus, O. plumifera and C. jobei.
The latter species are widespread mainly in
waters with smaller oscillations of the physicochemical parameters.
During late spring-summer, the copepod
population structure changes signifIcantly in
relation to seasonal changes in the dominance
ratio among A. clausi, P. parvus and T. stylifera.
Their distribution patterns show a certain degree
of temporal segregation of abundance peaks,
which would reduce the negative aspects of interspecifIc competition. In summer, the spatial distribution of P. elongatus and T. longicornis, typical euryoedous species of estuarine environments and well adapted to higher temperature
and lower salinity (Hure et al.1980), assume particular importance. Their wide distribution in
the neritic system depends on both the interaction of several biological and hydrological factors
and their vertical migration pattern. In this season, the abundance of P. elongatus and T. longicornis is higher and their distribution range
extends from coastal to offshore zones and from
north to south (Hure et al. 1980). Their marked
spreading into the offshore waters is caused by
the very strong outflow of the Po River in this
period (Marchetti et al. 1985). In the neritic system, the change from coastal hypertrophic surface waters to offshore oligo-mesotrophic ones
could cause the vertical migration to the bottom
of the above-mentioned species, either in the
same water layer with high fluorescence values or
in the NAdDW current which flows southward.
These observations on the spatial distribution pattern of P. elongatus and T. longicornis
suggest some considerations on the circulation
of water masses of the western Adriatic neritic
system. These species could be considered as
hydrological indicators both of the coastal waters
flowing south along the western coastal zone
(Hure et al.1980) and of the NAdDW, which originates in the area between the Po delta and Istria
and flows deeply from north to south in the offshore zone of the neritic system (Russo et a1. in
press).
During winter, the zooplankton community
has a dearly different pattern of species composition than in summer. The low abundance and
biomass of zooplankton are mainly due to the
lack of Penilia avirostris, Acartia cla us i,
Paracalanus parvus and Temora stylifera.
Copepods are the dominant zooplankton group,
even though they strongly decrease in number.
Moreover, there is an increased abundance of
pelagic species. which have a wide vertical distribution and are typical of sub-surface waters of
the southern Adriatic Sea. Their spreading
toward north-central areas is mainly related to
the seasonal ontogenetic migration to the surface
(Hure et a1. 1980). In this period, lower sunlight
and temperature values, together with greater
water turbulence, determine a strong reduction
of the primary production. The low primary production resources (Fonda Umani et al. 1992) are
used by a high number of species, so that the
copepod population structure is characterized
by a more equal distribution among the species
of the neritic system. No species becomes so